First, all transformers make acoustic noise. That is their nature. But the manufacturer should know how to limit or dampen these vibrations to keep them pretty much inaudible.
The transformer in a power conditioner or power amplifier will make audible noise for primarily one reason: improper AC input. All transformers require a constant specified voltage and frequency just like all other electronics. The AC input requirements are specified on the rear of the unit.
If one feeds the unit improper voltage or frequency, the magnetized plates in the core of the transformer do not operate properly and rattle
hence the noise. And this information comes from the chief engineer at Powervar.
Powervar and many other power conditioners do not regenerate the AC sine wave or regulate voltage or frequency. The AC that goes in comes out less AC TRANSIENTS and AC NOISE.
With that being said, there are many AC line sources that can cause transformer noise. If you are interested in understanding what they are and becoming educated on the subject, I suggested highly that you read the following article and focus on section 3. Adverse Mains Conditions. Go to: http://www.plitron.com/PDF/AES.PDF The article is excellent.
So if you are going to use a Powervar, you should feed it proper AC sign wave. The AC can be full of transients and noise but it has to be the correct voltage and frequency. The Powervar will clean the transients and noise but it is not going to change the sine wave. One can only do that with a regenerator and that comes with a price of course.
And I should state that the transformer in a conditioner can itself be damaged to the point it vibrates too much (audibly) regardless of whether the AC fed into it is proper or not.
